In 2011, an X-ray Free-Electron-Laser project was started in the Pohang Accelerator Laboratory (PAL-XFEL). The construction of the PAL-XFEL was finished at the end of 2015, and the commissioning was started from April 2016. The electron beam energy of 10 GeV was achieved at the end of April and the bunch compression was tried in May. The undulator commissioning was started from June. During the commissioning process, various kinds of instruments were used for the beam parameter monitoring including beam position monitors, beam profile monitors, beam charge monitors, beam arrival-time monitors, and beam loss monitors. This work will introduce the PAL-XFEL diagnostic system which was used in the commissioning process.